Two related areas of investigation of visual function are proposed. The first concerns the functional characterization of single cells of the retino-geniculate pathway of Old-World monkeys with regard to the receptive-field organization and interactions of the different cone signals. These investigations will make use of a color stimulator, designed for these studies, that can generate patterned stimuli able to be chromatically altered in any direction in color space. The aim of these studies is to understand the neural basis of color discrimination, in terms of the spatial and temporal characteristics of the contributions of the different cone types to the receptive fields of single units in the lateral geniculate nucleus and retina. The second area of investigation focuses on the morphologic features of this pathway. Two related studies are proposed, which will attempt to determine the different types of ganglion cells in the human retina, correlate them with those in Old-World monkey retinas, and determine the central projections of the monkey ganglion cells. The Golgi method will be used to determine the morphologic features of human and monkey ganglion cells, based upon material we now have and continue to acquire. This material also shows a number of different forms of amacrine cells, and we propose to investigate the morphologic features of this class of retinal cell as well. The central projections of cat and monkey ganglion cells will be investigated by a new method in which a localized central injection of a retrogradely-transported fluorescent dye will be used to identify those ganglion cells that project to a given terminal zone. The retina will be lightly fixed in a manner that does not disturb the integrity of the neuronal membrane. The labeled cells will then be iontophoretically injected with another fluorescent dye, so as to fill the entire dendritic tree. This will be done under direct visual control, using an extra-long working distance objective in an epifluorescent microscope.